426 THE POPULAR SCIENCE MONTHLY. 



them. But the mode of simplification adopted in physics and chemis- 

 try, viz., experiment, or the arranging of simple artificial conditions, is 

 only to a slight extent applicable to biology. The phenomena are not 

 only complex, but they are so delicately balanced that the introduc- 

 tion of our rude hands in the way of experiment often overthrows the 

 equilibrium, destroys the conditions of biological experiment, viz., life, 

 and thus throws the whole subject into the realm of chemistry and 

 physics. But, fortunately, nature has prepared for us an elaborate se- 

 ries of experiments. We have organisms of every degree of increasing 

 simplicity, from the body of man to the microscopic spherule of almost 

 unorganized protoplasm called a moner. The complex problem of life, 

 as we go down this scale, is made simpler by the successive removal of 

 added complications, until it is finally reduced to its simplest terms, 

 and thus only we begin to understand the essential phenomena — thus 

 only may we find the value of the unknown quantity. It is, therefore, 

 by extensive comparison of organisms in all stages of complication with 

 each other, that the foundations of a scientific biology have been laid. 

 Anatomy has become scientific only through comparative anatomy ; 

 physiology through comparative physiology; and, we may add, psy- 

 chology is now awaiting the development of comparative psychology. 



But this general method of comparison is subdivided into three or 

 four sub-methods. Nature has prepared not only one but three or four 

 series, not identical, not mere duplicates of each other, but varied, yet 

 resembling and illustrating each other. The first of these is the natural 

 history or taxonomic series. It consists of the whole series of organ- 

 isms as they now exist, from the complexly structured mammal to the 

 simple unicelled plant or animal. The second of these is the embrjonic 

 or ontogenic series. It consists of the successive stages of development 

 of one of the higher animals, from the germ-cell to the mature condition. 

 The third is the geological or phylogenic series. It consists of the 

 organisms inhabiting the earth in successive epochs, from the Archsean 

 until now. The fourth is the pathological series. It consists of all 

 possible variations from the normal type by monstrosity or by disease. 

 Though much less full than either of the others, it must not be neglected 

 by biologists. 



It is wholly by extensive comparison in these four series that biology 

 has recently risen to the rank of a true science. In this great work, the 

 chief credit is due to three men, viz., Cuvier, Agassiz, and Darwin ; for 

 these three are the great founders of the comparative method. Cuvier 

 laid the foundations of comparative anatomy and physiology, by com- 

 parison in the taxonomic series. Agassiz extended the comparison to 

 the ontogenic and phylogenic series, showed the resemblance between 

 the three, and determined and announced all i\ie formal laws of evolu- 

 tion of the oro;anic kinofdom as now recosrnized. Darwin has made the 

 bold and in large measure successful attempt to explain these laws by 

 the operation of secondary causes. If Agassiz may be called the Kepler 



